1. Field of the Invention
The present disclosure relates to a method of manufacturing a nitride semiconductor device.
2. Description of the Related Art
Gallium nitride (GaN) is a III-V compound semiconductor. A GaN compound semiconductor is used not only as an optical device such as a semiconductor laser and a light emitting diode (LED) operating in blue, green, and ultraviolet regions but also as a high temperature high output electronic device such as a high electron mobility transistor (HEMT) and a field effect transistor (FET) operating at a high temperature and a high output.
In general, when growing the GaN compound semiconductor to form a device, a hybrid substrate consisting of sapphire (Al2O3) or silicon carbide (SiC) is used. This is because it is difficult to commonly apply a bulk manufacturing method for a GaN substrate which requires high temperature and high pressure conditions in comparison with a melting method generally used for other semiconductors. Especially, a lattice constant and a thermal expansion coefficient of the Al2O3 substrate or the SiC substrate are largely different from those of GaN, which may cause a high defect density to a grown GaN crystal.
Such defects may reduce the efficiency in embodying the device and cause a leakage current. Accordingly, performance and yield of the device may be reduced. To this end, a high-quality single crystalline GaN substrate is demanded.
According to a commonly used method of manufacturing a single crystalline GaN substrate, a bulk GaN layer is grown on a hybrid substrate such as the Al2O3 substrate or the SiC substrate by a hydride vapor phase epitaxy (HVPE) method, and then the Al2O3 substrate or the SiC substrate is removed. Here, a laser lift-off process may be applied in removing a lower substrate such as the Al2O3 substrate or the SiC substrate. However, since the laser lift-off process generates high heat from a laser beam, the GaN substrate may be affected by the heat. In other words, the laser lift-off process may cause a stress between a nitride semiconductor layer and the lower substrate such as the Al2O3 substrate or the SiC substrate, thereby even breaking or damaging the GaN substrate.
In manufacturing a light emitting device such as an LED or a laser diode (LD), a hybrid substrate consisting of a different material from GaN, such as silicon, Al2O3, and SiC, is generally used. However, when a GaN-based material is grown on the hybrid substrate, defects such as a lattice mismatch or threading dislocation may occur in the grown thin film due to a difference in thermal expansion coefficients and a difference in crystalline lattice constants.
In a nitride semiconductor LED, Al2O3 is generally used for a substrate. Since the Al2O3 does not conduct an electric current, electrodes for supply of currents are laterally arranged. Here, part of light generated from an active layer escapes to the outside, thereby influencing the external quantum efficiency. However, actually, lots of light disappears in the form of heat, being shut in the Al2O3 substrate and the nitride semiconductor layer. In addition, since a current is applied in a lateral direction, imbalance of a current density occurs in the light emitting device, thereby deteriorating the performance of the device.
To this end, researches are underway to develop a technology for manufacturing a light emitting device where the Al2O3 substrate is removed and the electrodes have a vertical structure. Generally, a laser process may be used to remove the Al2O3 substrate. However, the laser process usually induces stress between the Al2O3 substrate and the nitride semiconductor that is the light emitting device. Therefore, the nitride semiconductor may be damaged.